Theoretical Study Of High-order Harmonic Generation From H₂⁺ Under The Two-dimensional Condition

With the rapid development of laser technology,the pulse width is decreased and the laser intensity is increased.The high-order harmonics generated by the interaction of laser with atoms,molecules and solids provide a very important technical way to study the quantum states of atoms,molecules and solids.In this thesis,we numerically solve the time-dependent Schr（?）dinger equation in two-dimensional condition to simulate the generation of high-order harmonics from molecular ions.With the H₂⁺as the model,the path control and the minima in harmonic spectrum are studied,and the main research are as follow:（1）Controlling paths of high-order harmonic generation from H₂⁺is theoretically investigated by changing frequency of the laser field perpendicular to molecular axis.Multiple returns are suppressed and short paths are dominant in the process of harmonic emission by two-dimensional orthogonalω/2ωlaser fields.Furthermore,not only are multiple returns weaken,but also the harmonic emission varies from twice to once in an optical cycle by orthogonalω/1.5ωlaser fields.Combining the time-frequency distributions and the time-dependent electron wave packets probability density,the simulations show that the change of harmonic emission paths is dependent on time-dependent electronic distribution.As a result,a68-as isolated attosecond pulse is obtained by superposing a proper range of the harmonics.（2）The mechanism of the minima in harmonic spectra from H₂⁺with large nuclear distances is theoretically investigated.For small nuclear distance（eg:R=4 a.u.）,there is no minimum in the harmonic spectra.However,the harmonic spectra exhibit clear minima with the increase of nuclear distance.Combining the time-frequency distributions,the classical kinetic energy map and population of electrons in the ground and excited states,the simulations show that minima in large nuclear distances are dependent on the back and forth transition of electrons between the ground state and excited state.Furthermore,by changing the orientation angle between the laser field and the molecular axis,the minima in large nuclear distances are effectively controlled.It further verifies the mechanism of the minima.